Synthesis, Performance And Mechanism Of Carbon-base Non-noble Oxygen Reduction Reaction Catalysts

Proton exchange membrane fuel cells(PEMFCs) that directly convert chemical energy into electrical energy can be applied to portable power and fuel cell electric vehicles, due to their advantages such as environment-friendliness, high power density and high convert efficiency. However, high loading of Pt-based catalysts and sluggish reaction kinetics on the cathode oxygen reduction reaction(ORR) are the main technical barriers in PEMFCs. The high price, resource shortage and easy poisoning of Pt hinder the commercial application of PEMFCs.Thus, developing inexpensive, high performance and durability non-noble metal cathodic catalysts will promote the large-scale commercialization of PEMFCs. Carbon-based non-noble ORR catalysts have been widely studied due to their ORR activity, anti-poisoning ability and excellent stability. However, carbon-based non-nobel ORR catalysts still cannot meet the requirements because of their much lower ORR activity in acidic media than Pt/C. Moreover, the active center of carbon-based catalysts is still controversial. Many factors, such as metal filling, edge carbon, specific surface area and so on, will affect the ORR performance of carbon-based catalysts.In this thesis, a series of carbon-based non-nobel ORR catalysts have been designed and prepared base on the edge carbon, heteroatom doping and iron filling. The ORR active sites and the catalytic mechanism of these catalysts have been discussed in detail. The main contents are summarized as follows:(1)The preparation of carbon nanotubes with increasing content of edge carbons and their electrocatalytic mechanism for ORR.Fish-bone carbon nanotubes(F-CNTs) and parallel carbon nanotubes(P-CNTs) without doped heteroatom were selected as research object. A series of CNTs with different contents of edge carbon atoms were obtained by H2, HNO3, ballmilling and annealing post-treatments. The results show that the more edge carbon the surface of CNTs has, the better ORR catalytic activity the CNTs exhibit. The ORR peak potential of CNTs is heavily dependent on the fraction of edge carbon atoms that was determined by electron transfer rate constant. The DFT calculation demonstrates that the high ORR catalytic activity of edge sites originates from the electron transfer between edge carbon atom and its neighbour carbon, leading to easier O2 adsorption.Chemically drilling as a controllable way was employed to make CNTs expose more edge carbon atoms. A series of drilled CNTs by using different Co loadings were prepared. The optimal Co loading for drilled CNTs is 10-12 wt%. The optimal sample exhibits high onset potential of-0.027 V(vs Ag/AgCl, the same below) and large limiting current of 4.58 mA cm-2, which is much better ORR catalytic activity than CNTs and N-doped CNTs, and is comparable to Pt/C in alkaline medium. TEM, Raman and EIS show that the combined effect of the edge carbon atoms and the electron transfer resistance determines the ORR activity of the drilled CNTs. This study provides a simple strategy to enhance the ORR activity of nondoped carbon material by exposing more edge carbon atoms.(2)The preparation, performance and their acitive sites of P-N, S-N and B-N dual-doped few-layered carbon nanosheets as electrocatalyst for ORR. P-N, S-N and B-N dual-doped fewlayered carbon nanosheets were prepared by pyrolysing chitosan, urea, phosphoric acid, sulfuric acid and boric acid, respectively. These prepared carbon nanosheets were graphenelike material with large BET surface area(~1000 m2 g-1), approximate graphitization and defective degree. The dual-doped carbon nanosheets exhibit significantly superior ORR onset potential(0.653 V), current(4-5.5 mA cm-2) and electron transfer number(~3.9) than N-doped CNTs, and their ORR currents are even larger than Pt/C in acidic medium. The doped heteroatom was analyzed carefully by XPS survey spectra. It was found that the graphitic nitrogen may be the most important catalytic active site for ORR.(3)Nitrogen doped carbon nanotubes(NCNTs) with encapsulated ferric carbide as excellent electrocatalyst(Fe3C@NCNTs) for ORR in acid and alkaline media and its electrocatalytic mechanism. Fe3C@NCNTs was synthesized by a simple direct pyrolysis of melamine and ferric chloride. The typical Fe3C@NCNTs is composed of uniform nanotubes with diameters of 100-200 nm and with filled Fe3 C nanoparticles in the interior and doped N on the surface. The as-prepared Fe3C@NCNTs catalyst exhibits the same ORR activity as Pt/C; in acid media its onset potential, limiting current and electron transfer number for ORR are 0.719 V, 3.6 mA cm-2 and 3.9, respectively; in alkaline media its onset potential, limiting current and electron transfer number for ORR are 0.098 V, 3.1 mA cm-2 and 4.0, respectively. Fe3C@NCNTs also exhibits superior methanol tolerance and stability to Pt/C. After 10,000 cyclic voltammograms, the ORR peak currents of Fe3C@NCNTs remain approximate 85.4% and 95.5%, respectively, in acid and alkaline media. XPS, poisoning and acid leaching experiments proved that the doped N is the main active site for ORR and the inner Fe3 C with outside carbon form the synergetic active site to enhance the ORR activity. A direct four electron transfer pathway was determined in acid media for ORR.